issue 5 march 2014 astropah
TRANSCRIPT
Issue 5 | March 2014
AstroPAHA Newsletter on Astronomical PAHs
EditorialDear Colleagues,
This is the 5th release of AstroPAH. This month a lot is going on. We start with the Picture ofthe Month, showing an illustration of PAHs on Titan. Two abstracts presented in this issue con-tain exciting new results about PAHs on Titan, where they appear to be precursors to aerosols.Another abstract discusses PAHs on Enceladus, another of Saturns moons. This at a time thatKepler has made head lines again, finding more than 715 new planets. And don’t miss any ofthe other abstracts – In this issue there are works on graphene etching, photochemistry, PAHsin the ISM, and a major update of the NASA Ames database.
Concerning news has reached us about the Stratospheric Observatory for Infrared Astron-omy, SOFIA. It seems that uncertain times are ahead. We would like to have some input onthis subject from the PAH community. Please tell us, for example, if the possible grounding ofSOFIA would have impact in your present or future projects or your opinion on this matter.
On other news, we are proud to announce that our editor Ella Sciamma-OBrien has receivedthe Outstanding Early Career Space Scientist award from the Space Science and AstrobiologyDivision at NASA Ames Research Center. Congratulations to Ella!
In Focus this issue is the European Task Force for Laboratory Astrophysics (ETFLA). Weencourage you to take a look and subscribe to the LABASTRO mailing lists if you have aninterest in or are connected to laboratory astrophysics.
Finally, please send us your contributions to AstroPAH any time – abstracts, suggestions forIn Focus, awards, future events, or other information you would like to share with the community.The deadline for contributions to appear in the next issue is 4 April 2014. The next issue ofAstroPAH will be out on 15 April 2014.
For more information on AstroPAH, visit our website:
http://astropah-news.strw.leidenuniv.nl.
Best regards
The Editorial Team
2 AstroPAH - March 2014 | Issue 5
AstroPAH Newsletter
EDITORIAL BOARD:
Editor-in-ChiefProf. Alexander TielensLeiden Observatory (The Netherlands)
Executive EditorDr. Isabel AlemanLeiden Observatory (The Netherlands)
EditorDr. Alessandra CandianLeiden Observatory (The Netherlands)
EditorDr. Elisabetta MicelottaInstitut d’Astrophysique SpatialeCNRS/Universite Paris-Sud (France)
EditorDr. Annemieke PetrignaniLeiden Observatory andRadboud University Nijmegen(The Netherlands)
EditorDr. Ella Sciamma-O’BrienNASA Ames Research Center (USA)
WEBSITE:http://astropah-news.strw.leidenuniv.nl
CONTACT:[email protected]
SUBSCRIBE:Click here to subscribe to AstroPAH
Contents
PAH Picture of the Month 1
Editorial 2
In Focus: European Task Forcefor Laboratory Astrophysics 4
Recent Papers 7
PAH Picture of the Month
The role of polycyclic aromatic hydrocar-bons (PAHs) in the formation of aerosols inTitan’s haze. PAHs appear to be precursorsto aerosols, triggering the first reactions thatcause these large, solid particles to sink intoTitan’s lower atmosphere. Credit: ESA / ATGMedialab
3
In Focus:Jonathan Tennyson Presents
the European Task Force for Laboratory Astrophysics(ETFLA)
”Astronomers observe light and other signals from deep space. However the interpretation ofthese signals requires a deep understanding of the physical processes involved in generat-ing them. This understanding can only be obtained on the back of laboratory investigations.Laboratory Astrophysics is there as an essential component in any successful astronomicalcampaign, and the understanding of extra-terrestrial PAHs is certainly no exception to this rule.
Recently the European-Union-funded ASTRONET pub-lished a roadmap for European Astronomy1. ASTRONET de-fined laboratory astrophysics as ”laboratory physics, chem-istry and biology, and theoretical calculations and modelling,of atomic, molecular, nuclear and solid state properties, pro-cesses and associated astrophysical phenomena that are re-quired to ensure the success of current and future researchprogrammes in ... astronomy”. The European Task Forcefor Laboratory Astrophysics (ETFLA) was appointed by AS-TRONET to report on the status of Laboratory Astrophysicsin Europe. Louis d’Hendecourt (Institut d’Astrophysique Spa-tiale, Universite Paris-Sud) and I jointly chair this exercise. Atthe same time, the American Astronomical Society has cre-
ated a Laboratory Astrophysics Division, and the American Chemical Society an astrochemistrysubdivision.
Laboratory astrophysics is clearly an essential support activity for wider astronomi-cal investigations; however, it is also a discipline in its own right. Laboratory astrophysicsprobes the properties of matter at all extremes: extremes of temperatures, extremes of pres-sure, and extremes of energy. This work is therefore intellectually challenging and driven bycuriosity about the fundamental behaviour of matter in environments not routinely encounteredon Earth. For example, the MATRI2CES setup, seen in the picture below, was developed in theSackler Laboratory for Astrophysics in Leiden to detect by high-resolution mass spectrometrythe reaction products resulting from the UV irradiation and/or H-atom bombardment of interstel-lar ice analogues.
1 The ASTRONET Infrastructure Roadmap: A Strategic Plan for European Astronomy. Editors: Michael F. Bode,Maria J. Cruz & Frank J. Molster, ISBN: 978-3-923524-63-1 (ASTRONET, 2008).
4 AstroPAH - March 2014 | Issue 5
Laboratory astrophysics research naturally leads to discoveries which impact on otherdisciplines as well as society, sometimes very significantly. A classic example of this isthe discovery of the first fullerene molecule, C60, by Kroto, Smalley and co-workers. The orig-inal 1985 experiment was performed in an attempt to explain interstellar medium absorptionsby diffuse interstellar bands. This work led to the foundation of a whole new discipline in thephysical sciences and the award of the 1996 Nobel Prize for Chemistry to its discoverers. Thiswork has had important extensions into material sciences including the development of carbonnanotubes, as well as the further discovery of graphene, for which the Nobel Prize for Physicswas awarded in 2010. 2010 also finally saw the detection of C60 in space. C60, and recently itsion C+
60, have now been identified in astrophysical sources ranging from planetary nebulae toyoung stellar objects bringing the story round a full circle.
While not all endeavours in laboratory astrophysics are as spectacularly successful as thediscovery of fullerenes, it is clear that the potential for this sort of break-through is inherent inthe discipline. ETFLA was asked to investigate the status of laboratory astrophysics in Europe.For this purpose we divided the discipline into a number of themes:
A. Gas phase astrochemistry (Interstellar Medium (ISM) and planetary atmospheres);B. Spectroscopy of the ISM;C. Spectroscopy in hot bodies (opacities);D. Solid state and molecular complexity;E. Stellar and planetary formation;F. Primitive and planetary material;G. High-energy processes and space plasmas;H. Stellar evolution and nuclear astrophysics;I. Astrophysical conditions for the emergence of life;
all of which are augmented by one underpinning theme:J. Databases.
5 AstroPAH - March 2014 | Issue 5
The ETFLA report, a preliminary version of which can be found on the new European labo-ratory astrophysics community website (www.labastro.eu), came to the following conclusions:
Laboratory astrophysics provides vital underpinning to observational astronomy aswell as being a vibrant and scientifically challenging discipline in its own right. Thereis considerable laboratory astrophysics activity in Europe, much of it world leading. However,the work is fragmented and, with the exception of the area of nuclear astrophysics, poorlycoordinated and patchily funded, and, in places, poorly recognised. To address these issueswe recommend:
A. Community building measures both within the laboratory astrophysics community itselfand between astronomers and laboratory astrophysics researchers working on commonthemes. These activities should form a natural part of any planning for future astronomicalmissions. For this we propose European-scale networking activities and fellowships whichwill allow young scientists to move between laboratory astrophysics and observational as-tronomy groups, and help attract other institutions to laboratory astrophysics.
B. 2% of all funding for major space missions and ground-based observatories should bededicated to supporting laboratory astrophysics activity.
C. Building on the work performed by the Virtual Atomic and Molecular Data Centre (VAMDC,www.vamdc.eu) consortium to provide access to data in all key areas of laboratory astro-physics. This work should be integrated with that of the Virtual Observatory (VO).
D. Working with publishers to ensure due recognition of data providers and to allow for betterintegration of laboratory astrophysics within the astronomy literature.
E. Raising the profile and visibility of European laboratory astrophysics.
F. Curacy of various Solar System material returned by space missions by establishment ofEuropean facilities.
To encourage networking we have created the new website mentioned above:
www.labastro.euThis site is still a work in progress; please email any comments, additionsor corrections to [email protected]. In addition we have opened anew [email protected] account that will address issues specificallylinked to laboratory astrophysics. If you want to register for the LABASTRO mail-ing list, please follow the instructions under ’email contact’ on the website.”
Jonathan Tennyson is Massey Professor ofPhysics in the Department of Physics andAstronomy of University College London.
6 AstroPAH - March 2014 | Issue 5
Recent Papers
Dynamics of Hydrogen and Methyl Radical Loss from Ion-ized Dihydro-Polycyclic Aromatic Hydrocarbons: A TandemMass Spectrometry and Imaging Photoelectron-PhotoionCoincidence (iPEPICO) Study of Dihydronaphthalene andDihydrophenanthreneBrandi West1, Christine Joblin2,3, Valerie Blanchet4, Andras Bodi5, BalintSztaray6, Paul M. Mayer1
1 Chemistry Department, University of Ottawa, Ottawa, Canada K1N 6N52 Universite de Toulouse, UPS-OMP, IRAP, F-31028 Toulouse cedex 4, France3 CNRS, IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France4 Laboratoire des Collisions Agregats Reactivite, Universite Toulouse-CNRS, 31000 Toulouse, France5 Molecular Dynamics Group, Paul Scherrer Institut, Villigen 5232, Switzerland6 Chemistry Department, University of the Pacific, Stockton, California 95211, United States
Ionized 1,2-dihydronaphthalene (C10H+10) and 9,10-dihydrophenanthrene (C14H+
12) are homol-ogous dihydrogenated polycyclic aromatic hydrocarbons containing adjacent sp3 carbon sites.Tandem mass spectrometry involving kiloelectronvolt collision induced dissociation was em-ployed to aid in the structural characterization of the products of the main dissociation chan-nels, loss of H (and subsequent H and H2 losses in dihydronaphthalene) and CH3. Evident fromboth the CID mass spectra and the imaging photoelectron-photoion coincidence (iPEPICO)breakdown curves is the fact that there are two competitive routes to the loss of H. For 1,2-dihydronaphthalene these give activation energies of 2.22 ± 0.10 and 2.44 ± 0.05 eV, whereasonly 2.37 ± 0.12 eV was obtained for 9,10-dihydrophenanthrene. The two parallel H-loss chan-nels are believed to be the result of isomerization taking place to the methylindene ion and the9-methylfluorene ion for 1,2-dihydronaphthalene and 9,10-dihydrophenanthren, respectively.Each newly formed isomer dissociates by H loss (one of the two competing H-loss reactions)and, of course, methyl loss. Methyl radical loss has similar kinetics for the two systems, E0 =2.57 ± 0.12 eV, ∆‡S = 18 ± 11 J K−1 mol−1 for ionized dihydronaphthalene and E0 = 2.38 ±0.15 eV, ∆‡S = -3 ± 15 J K−1 mol−1 for ionized dihydrophenanthrene, but as can be seen, theE0 and ∆‡S are slightly lower for the latter. The final bond rupture step in both H and CH3 lossis expected to be accompanied by a positive ∆‡S, thus the low energy H loss and CH3 loss orig-inate from the isomer ion in both cases, with the entropy of activation being dominated by theisomerization step. In contrast, sp3-H loss from the dihydro-PAHs differ by little in both systems(E0 = 2.44 eV in ionized dihydronaphthalene and 2.37 eV in ionized dihydrophenanthrene andthe ∆‡S values are 27 and 18 J K−1 mol−1, respectively). The presence of a second sp3 carbonsite has decreased the C-H bond dissociation energy relative to protonated naphthalene and
7 AstroPAH - March 2014 | Issue 5
protonated phenanthrene, possibly to facilitate the restoration of the unaltered PAH ion. The cal-culated dihedral angle is -34.3◦ in C10H +•
10 whereas C14H +•12 has an angle of -49.6◦, indicating
that to restore the planar nature of the molecules, which is required for all reaction channelsinvestigated, there is more rearrangement needed for 9,10-dihydrophenanthrene. Energeticsand entropic values associated with H and H2 loss from [M - H]+ ions from ionized dihydronaph-thalene were determined to be 2.72 eV, 9 ± 17 J K−1 mol−1, and 2.85 eV, 9 ± 7 J K−1 mol−1,respectively.
E-mail: [email protected] by The Journal of Physical Chemistry Ahttp://pubs.acs.org/doi/abs/10.1021/jp500430g
The role of benzene photolysis in Titan haze formation
Y. Heidi Yoona, Sarah M. Horsta, Raea K. Hicksa,b, Rui Lia,c, Joost A. deGouwa,c, Margaret A. Tolberta,b
a Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, UCB 216, Boul-der, CO 80309, USAb Department of Chemistry and Biochemistry, University of Colorado at Boulder, UCB 216, Boulder, CO 80309,USAc Chemical Sciences Division, National Oceanic and Atmospheric Administration, Earth System Research Labo-ratory, Boulder, CO 80305, USA
During the Cassini mission to the saturnian system, benzene (C6H6) was observed through-out Titans atmosphere. Although present in trace amounts, benzene has been proposed to bean important precursor for polycyclic aromatic hydrocarbon formation, which could eventuallylead to haze production. In this work, we simulate the effect of benzene in Titans atmospherein the laboratory by using a deuterium lamp (115-400 nm) to irradiate CH4/N2 gas mixturescontaining ppm-levels of C6H6. Proton-transfer ion- trap mass spectrometry is used to detectgas-phase products in situ. HCN and CH3CN are identified as two major gases formed from thephotolysis of 2% CH4 in N2, both with and without 1 ppmv C6H6 added. Inclusion of benzenesignificantly increases the total amount of gas-phase products formed and the aromaticity of theresultant gases, as shown by delta analysis of the mass spectra. The condensed phase prod-ucts (or tholins) are measured in situ using high-resolution time-of-flight aerosol mass spectrom-etry. As reported previously by Trainer et al. (Trainer, M.G., Sebree, J.A., Yoon, Y.H., Tolbert,M.A. [2013]. Astrophys. J. 766, L4), the addition of C6H6 is shown to increase aerosol mass,but decrease the nitrogen incorporation in the organic aerosol. The pressure dependence ofaerosol formation for the C6H6/CH4/N2 gas mixture is also explored. As the pressure decreases,the %N by mass in the aerosol products decreases.
E-mail: [email protected] 233 (2014) 233-241http://dx.doi.org/10.1016/j.icarus.2014.02.006
8 AstroPAH - March 2014 | Issue 5
Nuclear star formation activity and black hole accretion innearby Seyfert galaxiesPilar Esquej1,2,3, Almudena Alonso-Herrero2,4, Omaira Gonzalez-Martın5,6, Se-bastian F. Honig7,8, Antonio Hernan-Caballero2,3, Patrick F. Roche9, CristinaRamos Almeida5,6, Rachel E. Mason10, Tanio Dıaz-Santos11, Nancy A. Levenson12,Itziar Aretxaga13, Jose Miguel Rodrıguez Espinosa5,6 and Christopher Packham14
1 Centro de Astrobiologıa, INTA-CSIC, Villafranca del Castillo, 28850, Madrid, Spain2 Instituto de Fısica de Cantabria, CSIC-Universidad de Cantabria, 39005 Santander, Spain3 Departamento de Fısica Moderna, Universidad de Cantabria, Avda. de Los Castros s/n, 39005 Santander, Spain4 Augusto G. Linares Senior Research Fellow5 Instituto de Astrofısica de Canarias (IAC), C/Vıa Lactea, 38205, La Laguna, Spain6 Departamento de Astrofısica, Universidad de La Laguna, 38205, La Laguna, Spain7 UCSB Department of Physics, Broida Hall 2015H, Santa Barbara, CA, USA8 Institut fur Theoretische Physik und Astrophysik, Christian-Albrechts-Universitat, 24098 Kiel, Germany9 Department of Physics, University of Oxford, Oxford OX1 3RH, UK10 Gemini Observatory, Northern Operations Center, 670 N. A’ohoku Place, HI 96720, USA11 Spitzer Science Center, 1200 East California Boulevard, Pasadena, CA 91125, USA12 Gemini Observatory, Casilla 603, La Serena, Chile13 Instituto Nacional de Astrofısica, Optica y Electronica (INAOE), Aptdo. Postal 51 y 216, 72000 Puebla, Mexico14 Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX 78249, USA
Recent theoretical and observational works indicate the presence of a correlation betweenthe star formation rate (SFR) and the active galactic nuclei (AGN) luminosity (and, therefore,the black hole accretion rate, MBH) of Seyfert galaxies. This suggests a physical connectionbetween the gas forming stars on kpc scales and the gas on sub-pc scales that is feedingthe black hole. We compiled the largest sample of Seyfert galaxies to date with high angu-lar resolution (∼0.4 ′′- 0.8 ′′) mid-infrared (8 - 13µm) spectroscopy. The sample includes 29Seyfert galaxies drawn from the AGN Revised Shapley-Ames catalogue. At a median distanceof 33 Mpc, our data allow us to probe nuclear regions on scales of ∼65 pc (median value).We found no general evidence of suppression of the 11.3µm polycyclic aromatic hydrocarbon(PAH) emission in the vicinity of these AGN, and used this feature as a proxy for the SFR. Wedetected the 11.3µm PAH feature in the nuclear spectra of 45% of our sample. The derivednuclear SFRs are, on average, five times lower than those measured in circumnuclear regionsof 600 pc in size (median value). However, the projected nuclear SFR densities (median valueof 22M� yr−1 kpc−2) are a factor of 20 higher than those measured on circumnuclear scales.This indicates that the SF activity per unit area in the central ∼65 pc of Seyfert galaxies is muchhigher than at larger distances from their nuclei. We studied the connection between the nu-clear SFR and MBH and showed that numerical simulations reproduce fairly well our observedrelation.
E-mail: [email protected], 780, 86 (2014)http://adsabs.harvard.edu/abs/2014ApJ...780...86E
9 AstroPAH - March 2014 | Issue 5
A molecular dynamics study on slow ion interactions withthe polycyclic aromatic hydrocarbon molecule anthraceneJ. Postma1, R. Hoekstra1, A. G. G. M. Tielens2, and T. Schlatholter1
1 KVI Atomic and Molecular Physics, University of Groningen, Zernikelaan 25 NL-9747 AA Groningen, The Nether-lands2 Leiden Observatory, Niels Bohrweg 2, NL-2333 CA Leiden, The Netherlands
Atomic collisions with polycyclic aromatic hydrocarbon (PAH) molecules are astrophysicallyparticularly relevant for collision energies of less than 1 keV. In this regime, the interaction dy-namics are dominated by elastic interactions. We have employed a molecular dynamics simu-lation based on analytical interaction potentials to model the interaction of low energy hydrogenand helium projectiles with isolated anthracene (C14H10) molecules. This approach allows for avery detailed investigation of the elastic interaction dynamics on an event by event basis. Fromthe simulation data the threshold projectile kinetic energies above which direct C atom knockout sets in were determined. Anthracene differential energy transfer cross sections and total(dissociation) cross sections were computed for a wide range of projectile kinetic energies. Theobtained results are interpreted in the context of PAH destruction in astrophysical environments.
E-mail: [email protected], 783, 61 (2014)http://iopscience.iop.org/0004-637X/783/1/61/
The NASA Ames PAH IR Spectroscopic Database Version2.00: Updated Content, Web Site, and On(Off)line ToolsC. Boersma1, C. W. Bauschlicher, Jr.2, A. Ricca2,3, A. L. Mattioda1,3, J. Cami3,4,E. Peeters3,4, F. Sanchez de Armas3, G. Puerta Saborido3, D. M. Hudgins5,and L. J. Allamandola1
1 NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035, USA2 NASA Ames Research Center, MS 230-3, Moffett Field, CA 94035, USA3 SETI Institute, 189 Bernardo Avenue 100, Mountain View, CA 94043, USA4 Department of Physics and Astronomy, PAB 213, The University of Western Ontario, London, ON N6A 3K7,Canada5 NASA Headquarters, MS 3Y28, 300 E St. SW, Washington, DC 20546, USA
A significantly updated version of the NASA Ames PAH IR Spectroscopic Database, the firstmajor revision since its release in 2010, is presented. The current version, version 2.00, con-tains 700 computational and 75 experimental spectra compared, respectively, with 583 and 60in the initial release. The spectra span the 2.5-4000µm (4000-2.5 cm−1) range. New tools areavailable on the site that allow one to analyze spectra in the database and compare them withimported astronomical spectra as well as a suite of IDL object classes (a collection of programs
10 AstroPAH - March 2014 | Issue 5
Figure 1: Several screenshots of the database (web) app, demonstrating its scope and tools.
utilizing IDL’s object-oriented programming capabilities) that permit offline analysis called theAmesPAHdbIDLSuite. Most noteworthy among the additions are the extension of the computa-tional spectroscopic database to include a number of significantly larger polycyclic aromatic hy-drocarbons (PAHs), the ability to visualize the molecular atomic motions corresponding to eachvibrational mode, and a new tool that allows one to perform a non-negative least-squares fit ofan imported astronomical spectrum with PAH spectra in the computational database. Finally, amethodology is described in the Appendix, and implemented using the AmesPAHdbIDLSuite,that allows the user to enforce charge balance during the fitting procedure.
E-mail: [email protected], 211, 8 (2014)http://iopscience.iop.org/0067-0049/211/1/8/article
See also the Press Release
Need to Track Organic Nano-Particles Across the Universe? NASA’s Got an App for That
11 AstroPAH - March 2014 | Issue 5
Graphene etching on SiC grains as a path to interstellarpolycyclic aromatic hydrocarbons formation
P. Merino1, M. Svec2, J.I. Martinez3, P. Jelinek2, P. Lacovig4, M. Dalmiglio4, S.Lizzit4, P. Soukiassian5,6, J. Cernicharo1, and J.A. Martin-Gago1,3
1 Centro de Astrobiologa INTA-CSIC, Carretera de Ajalvir, km.4, ES-28850 Madrid, Spain2 Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, CZ-16200 Prague, CzechRepublic3 Instituto Ciencia de Materiales de Madrid-CSIC, c/. Sor Juana Ines de la Cruz, 3, ES-28049 Madrid, Spain4 Elettra-Sincrotrone Trieste S.C.p.A., Area Science Park, S.S. 14, Km 163.5, I-34149 Trieste, Italy5 Commissariat a l’Energie Atomique et aux Energies Alternatives, SIMA, DSM-IRAMIS-SPEC, Bat. 462, 91191Gif sur Yvette, France6 Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, 91192 Gif sur Yvette, France
Polycyclic aromatic hydrocarbons as well as other organic molecules appear among the mostabundant observed species in interstellar space and are key molecules to understanding theprebiotic roots of life. However, their existence and abundance in space remain a puzzle. Herewe present a new top-down route to form polycyclic aromatic hydrocarbons in large quantitiesin space. We show that aromatic species can be efficiently formed on the graphitized surfaceof the abundant silicon carbide stardust on exposure to atomic hydrogen under pressure andtemperature conditions analogous to those of the interstellar medium. To this aim, we mimic thecircumstellar environment using ultra-high vacuum chambers and investigate the SiC surface byin situ advanced characterization techniques combined with first-principles molecular dynamicscalculations. These results suggest that top-down routes are crucial to astrochemistry to explainthe abundance of organic species and to uncover the origin of unidentified infrared emissionfeatures from advanced observations.
Schematic representation of the formation pro-cess of interstellar PAHs proposed in the paper.In the vicinity of Red Giant stars, silicon carbide(SiC) condenses quickly into grains which maybe coated by layers of graphene. The carbona-ceous materials resulting from this process arereleased into space as PAHs.
12 AstroPAH - March 2014 | Issue 5
E-mail: [email protected], [email protected] Communications 5, Article number: 3054 (2014)http://www.nature.com/ncomms/2014/140121/ncomms4054/full/ncomms4054.html
See also the Press Releases
A New Theory Explaining the Origin of Hydrocarbon Molecules in Interstellar Space
Descubierto un Proceso para Formar Molculas en el Espacio (PDF, in spanish)
ScienceShot: How to Make Organic Chemicals From Stardust
Photochemistry of PAHs in cosmic water ice: The effect ofconcentration on UV-VIS spectroscopy and ionization effi-ciencySteven H. Cuylle1, Louis J. Allamandola2 and Harold Linnartz1
1 Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden Universtiy, P.O. Box 9513, 2300 RA Leiden, theNetherlands2 NASA Ames Research Center, Space Science & Astrobiology Division, MS 245-6, Moffett Field, CA 94035, USA
Context. Observations and models show that Polycyclic Aromatic Hydrocarbons (PAHs) areubiquitous in the interstellar medium. Like other molecules in dense clouds, PAHs accrete ontointerstellar dust grains, where they are embedded in an ice matrix dominated by water. In thelaboratory, mixed molecular ices (not containing PAHs) have been extensively studied usingFourier transform infrared absorption spectroscopy. Experiments including PAHs in ices havestarted, however, the concentrations used are typically much higher than the concentrationsexpected for interstellar ices. Optical spectroscopy offers a sensitive alternative.Aims. We report an experimental study of the effect PAH concentration has on the electronicspectra and the vacuum UV (VUV) driven processes of PAHs in water-rich ices. The goal is toapply the outcome to cosmic ices.Methods. Optical spectroscopic studies allow us to obtain in-situ and quasi real-time electronicsolid state spectra of two prototypical PAHs (pyrene and coronene) embedded in water ice un-der VUV photoprocessing. The study is carried out on PAH:H2O concentrations in the range of1:30000 to pure PAH, covering the temperature range from 12 to 125 K.Results. PAH concentration strongly influences the efficiency of PAH cation formation. At lowconcentrations, ionization efficiencies are over 60% dropping to about 15% at 1:1000. Increas-ing the PAH concentration reveals spectral broadening in neutral and cation PAH spectra at-tributed to PAH clustering inside the ice. At the PAH concentrations expected for interstellarices, some 10 to 20% may be present as cations. The presence of PAHs in neutral and ion formwill add distinctive absorption bands to cosmic ice optical spectra and this may serve as a toolto determine PAH concentrations.
13 AstroPAH - March 2014 | Issue 5
E-mail: [email protected]& A, 562, A22 (2014)http://www.aanda.org/articles/aa/abs/2014/02/aa22495-13/aa22495-13.html
The Influence of Benzene as a Trace Reactant in Titan AerosolAnalogsMelissa G. Trainer1, Joshua A. Sebree2, Y. Heidi Yoon3, Margaret A. Tolbert3,4
1 Planetary Environments Laboratory, Code 699, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA2 NASA Postdoctoral Program Fellow, Code 699, Goddard Space Flight Center, Greenbelt, MD 20771, USA3 Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Box 216 UCB,Boulder, CO 80309, USA4 Department of Chemistry and Biochemistry, University of Colorado at Boulder, Box 215 UCB, Boulder, CO 80309,USA
Approximate benzene mixing ratios asa function of altitude, as measured byCassini instruments. After Koskinen etal. (2011), with additional data fromVuitton et al. (2007), Cui et al. (2009,INMS), and Vinatier et al. (2010, CIRS).The aerosol profile is plotted with arbi-trary volume extinction values vs. al-titude for comparison of general ver-tical profiles. Aerosol data above 400km were taken from UVIS (Koskinen etal. 2011), and from 100 to 400 km fromCIRS at 15◦S (C. M. Anderson 2012, pri-vate communication, after Anderson &Samuelson 2011).
Benzene has been detected in Titans atmosphere by Cassini instruments, with concentra-tions ranging from sub-ppb in the stratosphere to ppm in the ionosphere. Sustained levels ofbenzene in the haze formation region could signify that it is an important reactant in the for-mation of Titans organic aerosol. To date, there have not been laboratory investigations toassess the influence of benzene on aerosol properties. We report a laboratory study on the
14 AstroPAH - March 2014 | Issue 5
chemical composition of organic aerosol formed from C6H6/CH4/N2 via far ultraviolet irradiation(120 – 200 nm). The compositional results are compared to those from aerosol generated bya more traditional Titan mixture of CH4/N2. Our results show that even a trace amount of C6H6
(10 ppm) has significant impact on the chemical composition and production rates of organicaerosol. There are several pathways by which photolyzed benzene may react to form largermolecules, both with and without the presence of CH4, but many of these reaction mechanismsare only beginning to be explored for the conditions at Titan. Continued work investigating theinfluence of benzene in aerosol growth will advance understanding of this previously unstudiedreaction system.
E-mail: [email protected], 766, L4 (2013)http://iopscience.iop.org/2041-8205/766/1/L4/article
Aromatic and aliphatic organic materials on Iapetus: Anal-ysis of Cassini VIMS data
Dale P. Cruikshanka, Cristina M. Dalle Orea,b, Roger N. Clarkc, Yvonne J.Pendletona
a NASA Ames Research Center, Moffett Field, CA 94035, United Statesb SETI Institute, Mountain View, CA 94043, United Statesc U.S. Geological Survey, Denver, CO 80225, United States
We present a quantitative analysis of the hydrocarbon and other organic molecular inventoryas a component of the low-albedo material of Saturn’s satellite Iapetus, based on a revisionof the calibration of the Cassini VIMS instrument. Our study uses hyperspectral data from amosaic of Iapetus’ surface (Pinilla-Alonso, N., Roush, T.L., Marzo, G.A., Cruikshank, D.P., DalleOre, C.M. [2011]. Icarus 215, 75-82) constructed from VIMS data on a close fly-by of thesatellite. We extracted 2235 individual spectra of the low-albedo regions, and with a clusteringanalysis tool (Dalle Ore, C.M., Cruikshank, D.P., Clark, R.N. [2012]. Icarus 221, 735-743),separated them into two spectrally distinct groups, one concentrated on the leading hemisphereof Iapetus, and the other group on the trailing. This distribution is broadly consistent with thatfound from Cassini ISS data analyzed by Denk et al. (Denk, T. et al. [2010]. Science 327, 435-439). We modeled the average spectra of the two geographic regions using the materials andtechniques described by Clark et al. (Clark, R.N., Cruikshank, D.P., Jaumann, R., Brown, R.H.,Stephan, K., Dalle Ore, C.M., Livio, K.E., Pearson, N., Curchin, J.M., Hoefen, T.M., Buratti,B.J., Filacchione, G., Baines, K.H., Nicholson, P.D. [2012]. Icarus 218, 831-860), and afterdividing the Iapetus spectrum by the model for each case, we extracted the resulting spectrain the interval 2.7 – 4.0 µm for analysis of the organic molecular bands. The spectra revealthe C—H stretching modes of aromatic hydrocarbons at ∼3.28 µm (∼3050 cm−1), plus fourblended bands of aliphatic —CH2— and —CH3 in the range ∼3.36 – 3.52 µm (∼2980 – 2840cm−1). In these data, the aromatic band, probably indicating the presence of polycyclic aromatichydrocarbons (PAH), is unusually strong in comparison to the aliphatic bands, as was found for
15 AstroPAH - March 2014 | Issue 5
Hyperion (Dalton, J.B., Cruikshank, D.P., Clark, R.N. [2012]. Icarus 220, 752-776; Dalle Ore,C.M., Cruikshank, D.P., Clark, R.N. [2012], op. cit.) and Phoebe (Dalle Ore, C.M., Cruikshank,D.P., Clark, R.N. [2012], op. cit.). Our Gaussian decomposition of the organic band regionsuggests the presence of molecular bands in addition to those noted above, specifically bandsattributable to cycloalkanes, olefinic compounds, CH3OH, and N-substituted PAHs, as well aspossible Hn-PAHs (PAHs with excess peripheral H atoms). In a minimalist interpretation of theGaussian band fitting, we find the ratio of aromatic CH to aliphatic CH2 + CH3 functional groupsfor both the leading and trailing hemispheres of Iapetus is∼10, with no clear difference betweenthem. In the aliphatic component of the surface material, the ratio CH2/CH3 is 4.0 on the leadinghemisphere and 3.0 on the trailing; both values are higher than those found in interstellar dustand other Solar System materials and the difference between the two hemispheres may bestatistically significant. The superficial layer of low-albedo material on Iapetus originated in theinterior of Phoebe and is being transported to and deposited on Iapetus (and Hyperion) in thecurrent epoch via the Phoebe dust ring (Tosi, F., Turrini, D., Coradini, A., Filacchione, G., andthe VIMS Team [2010]. Mon. Not. R. Astron. Soc. 403, 1113-1130; Tamayo, D., Burns, J.A.,Hamilton, D.P., Hedman, M.M. [2011]. Icarus 215, 260-278). The PAHs on Iapetus exist in aH2O-rich environment, and consequently are subject to UV destruction by hydrogenation onshort time-scales. The occurrence of this material is therefore consistent with the assertion thatthe deposition of the PAH-bearing dust is occurring at the present time. If the organic inventorywe observe represents the interior composition of Phoebe, we may be sampling the originalmaterial from a region of the solar nebula beyond Neptune where Phoebe formed prior to itscapture by Saturn (Johnson, T.V., Lunine, J.I. [2005]. Nature 435, 69-71).
E-mail: [email protected] 233 (2014) 306-315http://dx.doi.org/10.1016/j.icarus.2014.02.011
The reaction of the benzene cation with acetylenes for thegrowth of PAHs in the interstellar mediumPierre Ghesquiere1, Dahbia Talbi1, Amir Karton2
1 Laboratoire Univers et Particules Montpellier, UMR5299-CNRS-Universite de Montpellier II, 34095 MontpellierCedex 05, France2 School of Chemistry and Biochemistry, The University of Western Australia, M310, 35 Stirling Hwy, Crawley6009, Australia
We investigated the formation of the naphthalene cation from the successive addition of twoacetylenes on the benzene cation. A DFT approach was used for that purpose. The entrychannel corresponding to the addition of the acetylenes is the energetically highest part of theentire reaction pathway. It was recalculated using the ab initio W1-F12 and G4 thermochemicalprotocols. At these levels, the energetically highest point in the entry channel is shown to be justbelow the energy of the free reactants. This Letter shows that the formation of the naphthalenecation from benzene and acetylenes should proceed in space.
16 AstroPAH - March 2014 | Issue 5
E-mail: [email protected] Physics Letters 595-596C (2014), pp. 13-19http://dx.doi.org/10.1016/j.cplett.2014.01.040
AstroPAH Newsletter
http://[email protected]
Next issue: 15 April 2014Submission deadline: 4 April 2014
17 AstroPAH - March 2014 | Issue 5